Customers appreciate this, says Robison: "Look around us. We live in a desert." While KB officials won't discuss sales, they say that Water Smart homes cost the same as others (a 2,300-square-foot Renoir goes for around $230,000), and can save homeowners $350 in annual utility costs. KB embraces efficiency as a competitive advantage in a merciless market. Over on the Strip, the same thing is happening, with mega-resorts that once competed to be the most ostentatious now battling to be the most sustainable. Energy-efficient rooms may not matter to most customers, but the resorts are eager for conference and meeting bookings from corporate clients who relish blowing their own green-company horns. And then there are the greenbacks saved. "The accountants running spreadsheets on this stuff look at it and go, 'Holy crap, look at how much money we can save by doing a water recycling system,' " says Roberts, the architect.

When the Palazzo hotel, part of the Sands Resort, was completed in 2007, it was the biggest building in the U.S. – some 400,000 square feet larger than the Pentagon. The U.S. Green Building Council awarded the building a LEED Silver rating for its efficiency. Building green made sense, says Katarina Tesarova, Las Vegas Sands Corp. sustainability director, because Sands both developed and occupies the resort, and can thus realize efficiency payoffs over time. Generous incentives helped: A 2005 state law – later rewritten – gave developers as much as $3 in tax breaks for every $1 spent on green buildings, netting the Palazzo and the LEED-heavy City Center hundreds of millions in tax credits.

Tesarova, a lithe woman with giant brown eyes and a Russian accent, begins a tour of the building's green features deep in the four-level parking pit, where she points to a trio of tanks behind a chain-link fence. "It's not so sexy," she says, but it's important. The system takes nuisance groundwater ("I hate the term," she adds; "for me, no water is nuisance") that would have to be pumped out of the garage anyway, and treats it until it's suitable for irrigating, taking the hotel's landscaping "off the potable water grid." Down the street, the notorious Bellagio fountains operate on a similar system, making them a weird hybrid of excess and sustainability. Untreated nuisance water is used for the Palazzo's huge cooling towers, which keep the building inhabitable in summer – saving more than 46 million gallons annually. Pools are heated, meanwhile, with one of the biggest rooftop solar thermal arrays in the country.

Also in the not-sexy-but-important category is the waste-sorting center, located in the "back of the house," a sort of underground city with an army of workers – 8,000 for the whole resort – walking and driving forklifts down wide hallways, disappearing into and emerging from doorways that lead to restaurants or shops. In one cavernous room, people in blue jumpsuits sort through 50 tons of waste per day. Food waste uncontaminated with other garbage is sent to hog farms as feed; the rest is composted. The resort's recycling rate is about 60 percent, says Tesarova, meaning more than 10,000 tons of waste are diverted from the landfill per year. Fifty stories above the underground city, opulent rooms are outfitted with super-efficient LED architectural lights, low-flow plumbing fixtures and occupancy sensors that let the climate control know when to crank up.

And therein lies that old Vegas paradox again: No matter how great the LEED rating, the sheer scale of the operation renders any notion of sustainability somewhat absurd. Yet that same scale gives small actions, like switching out a lightbulb, a big impact. Exchange 50,000 lightbulbs for more efficient ones, and it makes a difference. Spread that across the MGM Grand and Caesars, the other mega-resorts with ambitious sustainability initiatives, and you have efficiency, super-sized.

Perhaps nothing embodies this paradox more dramatically than City Hall. The seven-story glass facade looms over downtown like a giant mirror, reflecting both a cloud-dappled sky and the 77-kilowatt forest of solar "trees" out front. The airy, hushed lobby resembles that of a Manhattan bank, complete with a metal sculpture that looks like blocks of blue ice embedded in the wall. Finished last year for about $146 million, the structure is LEED Gold certified and has yet more solar panels on the roof. Perrigo, who has an office on the top floor, says it's twice the size of the old City Hall but uses half the energy.

That's the catch. In order to move into a more efficient building, the city abandoned its previous headquarters, which, at just 40 years old, was hardly antiquated. According to a National Trust for Historic Preservation study, building from scratch can incur a big "carbon debt" that can take 40 years to repay with efficiency gains (and presumably a lot longer for a megalith like the Palazzo). "We've found that you can retrofit a building and achieve similar levels of efficiency as with a new build," says efficiency think tank Rocky Mountain Institute associate Michael Bendewald. And yet the bigger-and-newer-is-better ethos often prevails, whether it's with public buildings or Inspirada's efficient new homes. Only by consuming more, the twisted logic goes, can we consume less.

But in this and so many other ways, Vegas is just America, only more so. We have the world's most insatiable appetite, devouring twice as much water and energy per capita than even economic powerhouses like Germany. We have long hooked "sustainability" to consumption, perpetuating the very system at the root of our environmental ills even as we seek to lessen its impact. We'd rather build giant new solar installations in the desert than re-insulate our 20-year-old houses.

In this respect, Vegas, and for that matter every other Southwestern city, is at a crossroads. It can't really control its future growth. But as it crawls out of the housing bust, it can decide, through land-use zoning, urban planning and water-hookup restrictions, how future growth will happen. Will developers continue to gobble up more land on the urban fringes, build more highways and put more cars on them to get to those efficient new homes? Or will they repurpose what is already there, occupy the boarded-up buildings downtown, fill in empty lots with efficient high-rises and urban gardens, xeriscape emerald-green neighborhoods and do away with the need for cars?

Downtown Vegas might hold an answer. After visiting Perrigo at City Hall, I stroll around the area, which, after decades of decline, is being revitalized. I check out the bike lanes, painted Christmas-tree green for visibility, and look on as a sleek natural gas-powered bus glides past workers assembling the world's first shipping-container mall. And then I tour the new headquarters of the online retailer Zappos. The company moved from the Bay Area to suburban Henderson in 2004. When it outgrew those digs, it considered building a mega-corporate campus out on the city's edge. Instead, Zappos took over one of the empty buildings downtown, gutted it and refurbished it as an efficient, hipster-corporate HQ for 1,450 employees. The building? Las Vegas' former City Hall.

Jonathan Thompson is an HCN senior editor based in Durango, Colorado.